Capacity Loss

[miki]

I am testing a freezer prototype and have a problem with gradual capacity loss. It is 14 cu ft reach-in freezer, hermetic R-404A 1/2HP compressor, cap tube system. After start-up the freezer reaches –20F in approximatelly 1.5hr, goes into cyccling stage around 2cycle/hr, then number of cycles decreases, and after 4-5 days the temperature does not go below –16F, with tendency of even more capacity loss. We detected oil accumulation in the evaporator, modidifed suction piping and that seems to resolve the problem with oil accumulation. There was also a small leak on the high-pressure gauge, so the gauge has been changed but we still have gradual capacity loss which becomes noticable after 3-4 days. Any suggestions, what else might be a reson what capacity loss?

[jacob perkins]

defrost maybe
what type do you have?

[R12rules]

Oil return means velocity is right inside suction tubing.
It means your compressor has plenty of capacity to pull the gas back, and the oil with it.

It also means you have enough defrost cycles in order to "expell" the oil from inside the evaporator, where it would just looooove to hang out .... if you let it.

Oil return also means you have gravity on your side. Your compressor is down from the evaporator, is it not?


Bottom condensing unit means you have gravity on your side. But you have longer lines to feed.

Top condensing unit means you have more velocity on your side, due to the shorter lines.

[jacob perkins]

lack of complete defrosts is what I meant

[miki]

Electric defrost heater is used for defrost. Defrost is set to run every six hours, and evaporator coil is free of ice after defrost. Bottom condensing unit is employed. Suction line is 3/8 OD copper tubing. Evaporator coil is tube and fin type with 1/2OD copper tubing. Circuiting on the coil is designed for oil self-draining (refrigerant/oil flow is either horizontal or downflow). Compressor flow rate around 22lb/hr.

[rimek]

What synthetic oil are you using? What are suction and discharge pressures at bottom(-20°)? What are suction and discharge pressures at failure? What size and length of cap tube? When the leak was fixed, did you have a chance to run several cycles? If the leak is fixed, and if warming up the freezer to room temp, then turning back on- restarts the your cycle of pain; then it sure sounds like an oil-log, but -20°F isn't that cold, so I ask what is the oil? How are you determining refrigerant charge?

Edit: how far into the evap tube have you extended the cap tube end- should be 2-3".

[Edited by rimek on 12-23-2004 at 11:41 PM]

[hvacbear]

Do you have a crankcase heater?

[condenseddave]

Have you ever seen a grown man naked?

No seriously, have you?

[condenseddave]

We're going to need full temp and pressure measurements of the high and low sides of the unit, before, during and after defrost.

You say you have defrosts every 6 hours, but, for how long, and what type of termination?

I doubt it's defrost related, though.

I'd be inclined to think that there was a metering issue. What type of metering device, again? Cap tube, you say???????? HMMMMMMMMM.

[hvacbear]

I read walk-in not reach-in. Wierd to have a cap tube I thought.

[miki]

At 85-87F ambient, -20F cabinet temperature, the high side pressure is 250PSIG, low side 11PSIG. After 3-4 days, as the capacity goes down, pressures are around 220PSIG high, 8.5 PSIG low with (cabinet at -16F). The defrost is terminated by defrost temperature probe (40F termination temperature) which is pressed between coil fins at coldest spot. Defrost lasts around 8 minutes. I do not have recorded pressures before, during, and after defrost. As I said, defrost does not seem to be a cause of the problem since all ice melts and drains during defrost. The compressor oil is polyolester and comes with the compressor from the manufacturer. The cap tube is TC-50 (0.050" ID), 15.5 ft long, 13 oz charge of R-404A.

[condenseddave]

Superheat? Subcool?

That much of a pressure drop would lead me to look further for refrigerant leaks.

13 oz charge, even with 404a, means you can't afford to lose a drop, and then "top off" the existing charge. IF, that is, you want to be critical on temps/pressures, which you appear to want. That gas CAN fractionate, and that will only show up in a small unit like that.

If you had a couple of small leaks already, the very first thing you must do, is to make certain that there are no more, at all, then recover the charge, evacuate, and charge in "virgin" gas.

[bustedknuckle]

jeez that cap tube seems to be awful long ain't it? 16' is the max right? i would try to cross it to a shorter lenth with a different dia. that should help with the capacity loss i would think, but hey, i've been wrong before